BACKGROUND: Congenital myopathies are severe genetic diseases with a strong impact on patient autonomy and often on survival. A large number of patients do not have a genetic diagnosis, precluding genetic counseling and appropriate clinical management. Our objective was to find novel pathogenic variants and genes associated with congenital myopathies and to decrease diagnostic odysseys and dead-end.
METHODS: To identify pathogenic variants and genes implicated in congenital myopathies, we established and conducted the MYOCAPTURE project from 2009 to 2018 to perform exome sequencing in a large cohort of 310 families partially excluded for the main known genes.
RESULTS: Pathogenic variants were identified in 156 families (50%), among which 123 families (40%) had a conclusive diagnosis. Only 44 (36%) of the resolved cases were linked to a known myopathy gene with the corresponding phenotype, while 55 (44%) were linked to pathogenic variants in a known myopathy gene with atypical signs, highlighting that most genetic diagnosis could not be anticipated based on clinical-histological assessments in this cohort. An important phenotypic and genetic heterogeneity was observed for the different genes and for the different congenital myopathy subtypes, respectively. In addition, we identified 14 new myopathy genes not previously associated with muscle diseases (20% of all diagnosed cases) that we previously reported in the literature, revealing novel pathomechanisms and potential therapeutic targets.
CONCLUSIONS: Overall, this approach illustrates the importance of massive parallel gene sequencing as a comprehensive tool for establishing a molecular diagnosis for families with congenital myopathies. It also emphasizes the contribution of clinical data, histological findings on muscle biopsies, and the availability of DNA samples from additional family members to the diagnostic success rate. This study facilitated and accelerated the genetic diagnosis of congenital myopathies, improved health care for several patients, and opened novel perspectives for either repurposing of existing molecules or the development of novel treatments.

X-linked myotubular myopathy (XLMTM) is a rare congenital myopathy resulting from dysfunction of the protein myotubularin encoded by the MTM1 gene. XLMTM has a high neonatal and infantile mortality rate due to a severe myopathic phenotype and respiratory failure. However, in a minority of XLMTM cases, patients present with milder phenotypes and achieve ambulation and adulthood. Notable facial dysmorphia is also present.
We investigated the genotype-phenotype correlations in newly diagnosed XLMTM patients in a patients\' cohort (previously published data plus three novel variants, n = 414). Based on the facial gestalt difference between XLMTM patients and unaffected controls, we investigated the use of the Face2Gene application.
Significant associations between severe phenotype and truncating variants (p < 0.001), frameshift variants (p < 0.001), nonsense variants (p = 0.006), and in/del variants (p = 0.036) were present. Missense variants were significantly associated with the mild and moderate phenotype (p < 0.001). The Face2Gene application showed a significant difference between XLMTM patients and unaffected controls (p = 0.001).
Using genotype-phenotype correlations could predict the disease course in most XLMTM patients, but still with limitations. The Face2Gene application seems to be a practical, non-invasive diagnostic approach in XLMTM using the correct algorithm.

BACKGROUND: X-linked myotubular myopathy (XLMTM) is a rare, life-threatening congenital muscle disease caused by mutations in the MTM1 gene that result in profound muscle weakness, significant respiratory insufficiency, and high infant mortality. There is no approved disease-modifying therapy for XLMTM. Resamirigene bilparvovec (AT132; rAAV8-Des-hMTM1) is an investigational adeno-associated virus (AAV8)-mediated gene replacement therapy designed to deliver MTM1 to skeletal muscle cells and achieve long-term correction of XLMTM-related muscle pathology. The clinical trial ASPIRO (NCT03199469) investigating resamirigene bilparvovec in XLMTM is currently paused while the risk:benefit balance associated with this gene therapy is further investigated.
METHODS: Muscle biopsies were taken before treatment and 24 and 48 weeks after treatment from ten boys with XLMTM in a clinical trial of resamirigene bilparvovec (ASPIRO; NCT03199469). Comprehensive histopathological analysis was performed.
RESULTS: Baseline biopsies uniformly showed findings characteristic of XLMTM, including small myofibres, increased internal or central nucleation, and central aggregates of organelles. Biopsies taken at 24 weeks post-treatment showed marked improvement of organelle localisation, without apparent increases in myofibre size in most participants. Biopsies taken at 48 weeks, however, did show statistically significant increases in myofibre size in all nine biopsies evaluated at this timepoint. Histopathological endpoints that did not demonstrate statistically significant changes with treatment included the degree of internal/central nucleation, numbers of triad structures, fibre type distributions, and numbers of satellite cells. Limited (predominantly mild) treatment-associated inflammatory changes were seen in biopsy specimens from five participants.
CONCLUSIONS: Muscle biopsies from individuals with XLMTM treated with resamirigene bilparvovec display statistically significant improvement in organelle localisation and myofibre size during a period of substantial improvements in muscle strength and respiratory function. This study identifies valuable histological endpoints for tracking treatment-related gains with resamirigene bilparvovec, as well as endpoints that did not show strong correlation with clinical improvement in this human study.
BACKGROUND: Astellas Gene Therapies (formerly Audentes Therapeutics, Inc.).

UNASSIGNED: Ryanodine receptor 1 (RYR1)-related myopathies are a group of congenital muscle diseases caused by RYR1 mutations. These mutations may cause centronuclear myopathy, a congenital neuromuscular disorder characterized by clinical muscle weakness and pathological presence of centrally placed nuclei on muscle biopsy. Mutations in RYR2 cause ventricular arrhythmias that can be treated with flecainide; however, reports of ventricular arrhythmias in RYR1-related myopathies are rare. Herein we report a case of centronuclear myopathy with RYR1 mutations who exhibited frequent premature ventricular contractions (PVCs) and non-sustained ventricular tachycardia (NSVT), which was successfully treated with verapamil and flecainide.
UNASSIGNED: At 7 months, the patient presented neurological manifestations of hypotonia and delayed motor development. A skeletal muscle biopsy performed at age 4 years led to the diagnosis of centronuclear myopathy. At age 15 years, frequent PVCs and NSVT were identified on the electrocardiogram and 24 h Holter monitoring. Treatment with verapamil was initiated; however, it was not beneficial. Therefore, flecainide was added to the treatment, decreasing the frequency of PVCs and NSVT. Non-sustained ventricular tachycardia disappeared at the age of 21, and PVCs almost disappeared at the age of 22. Genetic testing revealed c.13216delG (p.E4406Rfs*35), c.14874G>C (p.K4958N), and c.9892G>A (p.A3298T) in RYR1, and the compound heterozygosity of variants was confirmed by analysis of the parents.
UNASSIGNED: This is the first report of ventricular arrhythmia associated with RYR1-related myopathy that was successfully treated with verapamil and flecainide. The combination of verapamil and flecainide may be a useful treatment option for ventricular arrhythmias in patients with RYR1-related myopathies.

X-linked myotubular myopathy (XLMTM), a centronuclear congenital myopathy secondary to pathogenic variants in the MTM1 gene encoding myotubularin, is typically recognized for its classic and severe phenotype which includes neonatal hypotonia, severe muscle weakness, long-term ventilator dependence, markedly delayed gross motor milestones with inability to independently ambulate, and a high neonatal and childhood mortality. However, milder congenital forms of the condition and other phenotypes are recognized. We describe a 6-year-old boy with a mild XLMTM phenotype with independent gait and no respiratory insufficiency even in the neonatal period. The child has a hemizygous novel splice site variant in the MTM1 gene (c.232-25A > T) whose pathogenicity was confirmed by cDNA studies (exon 5 skipping) and muscle biopsy findings. We also compared the phenotype of our patient with the few reported cases that presented a mild XLMTM phenotype and no respiratory distress at birth, and discussed the potential mechanisms underlying this phenotype such as the presence of residual expression of the normal myotubularin transcript.

Dynamin 2 (DNM2) is a ubiquitously expressed GTPase regulating membrane trafficking and cytoskeleton dynamics. Heterozygous dominant mutations in DNM2 cause centronuclear myopathy (CNM), associated with muscle weakness and atrophy and histopathological hallmarks as fiber hypotrophy and organelles mis-position. Different severities range from the severe neonatal onset form to the moderate form with childhood onset and to the mild adult onset form. No therapy is approved for CNM. Here we aimed to validate and rescue a mouse model for the moderate form of DNM2-CNM harboring the common DNM2 R369W missense mutation. Dnm2R369W/+ mice presented with increased DNM2 protein level in muscle and moderate CNM-like phenotypes with force deficit, muscle and fiber hypotrophy, impaired mTOR signaling, and progressive mitochondria and nuclei mis-position with age. Molecular analyses revealed a fiber type switch toward oxidative metabolism correlating with decreased force and alteration of mitophagy markers paralleling mitochondria structural defects. Normalization of DNM2 levels through intramuscular injection of AAV-shDnm2 targeting Dnm2 mRNA significantly improved histopathology and muscle and myofiber hypotrophy. These results showed that the Dnm2R369W/+ mouse is a faithful model for the moderate form of DNM2-CNM and revealed that DNM2 normalization after a short 4-week treatment is sufficient to improve the CNM phenotypes.

[This corrects the article DOI: 10.3389/fneur.2021.761636.].

Background: Congenital myopathy constitutes a heterogeneous group of orphan diseases that are mainly classified on the basis of muscle biopsy findings. This study aims to estimate the prevalence of congenital myopathy through a systematic review and meta-analysis of the literature. Methods: The PubMed, MEDLINE, Web of Science, and Cochrane Library databases were searched for original research articles published in English prior to July 30, 2021. The quality of the included studies was assessed by a checklist adapted from STrengthening the Reporting of OBservational studies in Epidemiology (STROBE). To derive the pooled epidemiological prevalence estimates, a meta-analysis was performed using the random effects model. Heterogeneity was assessed using the Cochrane Q statistic as well as the I 2 statistic. Results: A total of 11 studies were included in the systematic review and meta-analysis. Of the 11 studies included, 10 (90.9%) were considered medium-quality, one (9.1%) was considered low-quality, and no study was assessed as having a high overall quality. The pooled prevalence of congenital myopathy in the all-age population was 1.50 (95% CI, 0.93-2.06) per 100,000, while the prevalence in the child population was 2.73 (95% CI, 1.34-4.12) per 100,000. In the pediatric population, the prevalence among males was 2.92 (95% CI, -1.70 to 7.55) per 100,000, while the prevalence among females was 2.47 (95% CI, -1.67 to 6.61) per 100,000. The prevalence estimates of the all-age population per 100,000 were 0.20 (95% CI 0.10-0.35) for nemaline myopathy, 0.37 (95% CI 0.21-0.53) for core myopathy, 0.08 (95% CI -0.01 to 0.18) for centronuclear myopathy, 0.23 (95% CI 0.04-0.42) for congenital fiber-type disproportion myopathy, and 0.34 (95% CI, 0.24-0.44) for unspecified congenital myopathies. In addition, the prevalence estimates of the pediatric population per 100,000 were 0.22 (95% CI 0.03-0.40) for nemaline myopathy, 0.46 (95% CI 0.03-0.90) for core myopathy, 0.44 (95% CI 0.03-0.84) for centronuclear myopathy, 0.25 (95% CI -0.05 to 0.54) for congenital fiber-type disproportion myopathy, and 2.63 (95% CI 1.64-3.62) for unspecified congenital myopathies. Conclusions: Accurate estimates of the prevalence of congenital myopathy are fundamental to supporting public health decision-making. The high heterogeneity and the lack of high-quality studies highlight the need to conduct higher-quality studies on orphan diseases.

Centronuclear myopathies (CNM) are rare congenital disorders characterized by muscle weakness and structural defects including fiber hypotrophy and organelle mispositioning. The main CNM forms are caused by mutations in: the MTM1 gene encoding the phosphoinositide phosphatase myotubularin (myotubular myopathy), the DNM2 gene encoding the mechanoenzyme dynamin 2, the BIN1 gene encoding the membrane curvature sensing amphiphysin 2, and the RYR1 gene encoding the skeletal muscle calcium release channel/ryanodine receptor. MTM1, BIN1, and DNM2 proteins are involved in membrane remodeling and trafficking, while RyR1 directly regulates excitation-contraction coupling (ECC). Several CNM animal models have been generated or identified, which confirm shared pathological anomalies in T-tubule remodeling, ECC, organelle mispositioning, protein homeostasis, neuromuscular junction, and muscle regeneration. Dynamin 2 plays a crucial role in CNM physiopathology and has been validated as a common therapeutic target for three CNM forms. Indeed, the promising results in preclinical models set up the basis for ongoing clinical trials. Another two clinical trials to treat myotubular myopathy by MTM1 gene therapy or tamoxifen repurposing are also ongoing. Here, we review the contribution of the different CNM models to understanding physiopathology and therapy development with a focus on the commonly dysregulated pathways and current therapeutic targets.

Mutations in the gene encoding dynamin 2 (DNM2), a GTPase that catalyzes membrane constriction and fission, are associated with two autosomal-dominant motor disorders, Charcot-Marie-Tooth disease (CMT) and centronuclear myopathy (CNM), which affect nerve and muscle, respectively. Many of these mutations affect the pleckstrin homology domain of DNM2, yet there is almost no overlap between the sets of mutations that cause CMT or CNM. A subset of CMT-linked mutations inhibit the interaction of DNM2 with phosphatidylinositol (4,5) bisphosphate, which is essential for DNM2 function in endocytosis. In contrast, CNM-linked mutations inhibit intramolecular interactions that normally suppress dynamin self-assembly and GTPase activation. Hence, CNM-linked DNM2 mutants form abnormally stable polymers and express enhanced assembly-dependent GTPase activation. These distinct effects of CMT and CNM mutations are consistent with current findings that DNM2-dependent CMT and CNM are loss-of-function and gain-of-function diseases, respectively. In this study, we present evidence that at least one CMT-causing DNM2 mutant (ΔDEE; lacking residues 555DEE557) forms polymers that, like the CNM mutants, are resistant to disassembly and display enhanced GTPase activation. We further show that the ΔDEE mutant undergoes 2-3-fold higher levels of tyrosine phosphorylation than wild-type DNM2. These results suggest that molecular mechanisms underlying the absence of pathogenic overlap between DNM2-dependent CMT and CNM should be re-examined.